1. Field of the Invention
The invention generally relates to power converters and, more particularly, the invention relates to control circuits for various types of converters with parallel phased switches.
2. Description of the Related Art
Integrated circuits and other circuit elements within electronic devices commonly require an input voltage that is smaller than that supplied by a regulated D.C. power source. For example, an integrated circuit within a computer system may require a powering voltage of about 2.5 volts D.C. from a standard 3.3 volt regulated D.C. source. To that end, voltage regulators have been developed that reduce an input D.C. voltage to a preselected, lower voltage.
Many such prior art regulators, such as linear converters, have a relatively low efficiency (i.e., between about fifty and sixty five percent) when used in high current applications. Accordingly, when used in high current applications, linear converters typically require relatively large heat sinks to dissipate a large amount of heat produced by the voltage reduction process. This heat loss necessarily increases the operating cost of such converters. Moreover, the requirement of a heat sink increases the size and manufacturing cost of each converter.
The related art has responded to this efficiency problem by providing switching voltage converters, called xe2x80x9cbuck converters,xe2x80x9d that dissipate minimal heat in high current (as well as low current) applications. Buck converters typically utilize an inductor and a switch that cooperate to reduce the input voltage to a preselected output voltage.
For example, when a 3.3 volt input is reduced to a 1.8 volt output, the high side switch of a buck converter must be of very low resistance when in the on-state and may have to operate at a high duty cycle due to the small difference in input to output voltages. In response to such case, a buck converter with paralleled and phased high side switches was developed.
Other prior art, such as linear regulators, have too low an efficiency (i.e., about 50%) to be used for more than a few amps of output current without heavy losses and a need for external heat sinking, thereby increasing the size and cost of the converter.
Buck converters have been developed that operate in a high frequency switching mode in an attempt to minimize loss. The buck converter is usually at least 65% to 95% efficient. Buck converters usually incorporate at least one switch (e.g., MOSFET or transistor), one coil, and a filter capacitor on the output for noise reduction and good transient response. These components are usually controlled by an integrated circuit that sets the duty cycle of the switch according to load demands as sensed by a feed-back sense line connected from the load back to the integrated circuit controller. Thus the controller compensates for both load and input line variations to produce a constant output.
As load current demands have raised it becomes difficult to use only one switch because of limitations of the on-state resistance of the switch and its heat dissipating capability at high duty cycles. One proposed solution to this problem has been to put more switches in parallel, and another is to use more voltage reduction sections in a phased switching sequence called a xe2x80x9cmultiphase converter.xe2x80x9d Both of these solutions have serious drawbacks. Simply paralleling transistors is not a good solution because the transistors often do not share current equally, and the configuration produces a constantly heavy load on the gate driver circuit. The xe2x80x9cmultiphasexe2x80x9d converters require an additional inductor with each parallel switch, which increases the cost of the manufacturing the converter.
The prior art of the integrated control circuits for the buck converter can be classified according to the following main general types, in order of their chronological development: hysteretic mode, voltage mode, peak current mode, average current mode, xe2x80x9cv-squaredxe2x80x9d mode, and xe2x80x9cmultiphasexe2x80x9d control. There are many other control techniques that have been used from time-to-time, however, the ones mentioned above are the ones in most general use today.
Of these control methods there is a further classification as xe2x80x9csynchronousxe2x80x9d or non-synchronous. The term synchronous, for buck converters, refers to the use of a low-side switch from the input of the inductor to common, which is turned on whenever the high-side switch is turned off so that continuous current can be maintained through the inductor. The integrated circuit control for a synchronous circuit turns on the low-side switch at all times when the high-side switch is turned off. xe2x80x9cHigh-sidexe2x80x9d refers to the switch connected directly between the input power source and the input to the inductor while the low-side switch refers to the switch connected from the input of the inductor to common or xe2x80x9cground.xe2x80x9d
The non-synchronous type of control circuit does not provide a drive signal for the low side switch, depending instead on an external diode connected from common to the inductor input to maintain continuous current flow. Compensation is usually provided by resistive and capacitive elements connected externally to the controller IC (if the integrated circuit controller requires compensation for the gain-phase characteristics of the feedback loop of the converter).
One example of a controller is the pulse width modulator (PWM) controller, which usually provides the following functions: An error amplifier, which senses the converter output or load variations and amplifies the difference relative to a fixed reference voltage for further processing by the controller; A voltage reference provided either internally to the controller or externally programmed through a digital to analog converter (DAC) which is internal to the controller; Logic and/or analog circuitry for determining whether an external high-side switch should be turned on or off depending on the signal from the error amplifier; Gate drive circuitry to provide PWM pulses to the external high and low-side switches; A clock circuit and a ramp generator for comparing the error signal to the instantaneous value of the ramp voltage to generate the PWM gate drive signals; Various sense and logic circuits for protective functions such as detecting a fault condition (short circuit, over-voltage on input or output, under-voltage on input, over-current, etc.); and Circuitry to prevent cross-conduction from the high-side switch through the low-side switch directly to common under the undesirable condition where the on-time of the high-side switch overlaps the on-time of the low-side switch.
Accordingly, it would be advantageous to provide an integrated circuit control that can control more than one high-side phased switch feeding a single inductor. Conversely, the existing so-called xe2x80x9cmultiphasexe2x80x9d controllers require that a separate inductor be used for each phase, which is costly and usually not economic to implement except for very high current applications. It is therefore to such inventive integrated circuit control that the present invention is directed.
The present invention discloses an integrated circuit controller that controls at least two high-side switches in converters with forced current sharing accomplished by phasing the duty cycles of the switches. In an alternate embodiment of the present invention, the integrated circuit controller controls at least two phased low-side switches in a power converter. In yet another alternate embodiment of the present invention, the integrated circuit controller controls at least two high-side phased switches and at least two low-side phased switches in a power converter. The high or low-side switches are controlled so that each switch turns on for only a fraction of the effective duty cycle under steady state load conditions. For example, if under a certain operating condition, an effective overall duty cycle of 50% is required, each switch of the two paralleled phased switches, high-side or low-side, will only turn on at a 25% duty cycle. Therefore each separate switch will run at a lower stress and will run cooler if all other parameters are equal. Because each switch runs cooler, its on-resistance will be lower and the converter will have less internal loss and therefore higher efficiency. Alternatively, higher resistance switches could be used at a lower cost for the same overall efficiency.
In one embodiment, the integrated circuit controller is capable of driving both phased switches on at the same time for effective duty cycles of up to and including 100% in the event of emergency power demand or a heavy load transient, and then can return to the phased switch operation under normal steady state operation of the converter. For example, if a computer is suddenly forced to operate at a maximum data rate, its power demand may require emergency mode duty cycle from the source or on-board power converter in order to supply the increased power demand.
Inanother embodiment, the integrated circuit controller is capable of sustaining stable control of a converter with only one inductor output filter section even though two or more phased switches are controlled and forced to share current feeding into the inductor. This is because the phased switches are paralleled to act as one switch but forced to share current as required by load changes that require fast changes in duty cycle.
In accordance with other aspects of the invention, a number of different control schemes can be incorporated in the controller, such as voltage mode control, hysteretic control, average current mode, peak current mode, or v-squared control. This is because the method for controlling the paralleled phased switches works at the basic PWM modulation level of the error signal and does not otherwise depend on the exact detail control method
Preferably, even though high-side and/or low-side switches are phased, a low-side switch is prevented from being turned on during any time period when a high-side switch is turned on in order to prevent current cross-conduction from the power rail directly to common. Further, drive turn-on signal edges have appropriate delay circuits to prevent cross-conduction.
In a further embodiment of the invention, special PWM ramp signals are created that allow phasing of switches. Two clock signals are derived from a single source clock and drive the phased ramps. Other types of converters (other than buck converters) may be controlled with an alternate embodiment of the invention.
Other object, features, and advantages of the present invention will become apparent after review of the hereinafter set forth Brief Description of the Drawings, Detailed Description of the Invention, and the claims.